Histamine-Releasing Activity (HRA)

Abstract

We have recently reported the production of a histamine-releasing activity (HRA) by 1- to 2-day cultures of human mononuclear cells. Synthesis of HRA was augmented by addition of the mitogen concanavalin A or by antigens that commonly elicit cell-mediated immune responses. Since we have previously shown that histamine was released from basophils by activated complement, some of the properties of HRA and C5a were compared. No correlation between the percentage of histamine released from normal basophils by HRA or C5a was found. Anti-C5 abolished C5a-induced release, but did not inhibit the release of histamine from basophils by HRA. HRA did not resemble C3a: HRA released a higher percentage of histamine than C3a and HRA was not inhibited by anti-C3a. Pretreatment of basophils with HRA in the absence of calcium, followed by challenge with C5a, caused an augmented release of histamine by C5a. Basophils desensitized to C5a and anti-IgE released normally to HRA challenge. Basophils pretreated with HRA released normally to rechallenge with HRA.

Release of histamine from basophils by HRA was calcium-dependent and magnesium-independent. The maximal percentage of intracellular histamine was released within a few minutes after exposure to HRA. Optimal release occurred at 37°C.

HRA had an apparent m.w. of 10,000 to 15,000 daltons as determined by gel exclusion chromatography. HRA was relatively resistant to the effects of elevated temperature but was destroyed by trypsin and α-chymotrypsin. The effect of trypsin was prevented by soybean trypsin inhibitor. Nucleases and neuraminidase had no effect on HRA.